In the days after the Las Vegas shooting that killed 58 people, authorities said gunman Stephen Paddock acted alone. But search warrants, released after a court challenge by media including The Associated Press, showed that police and the FBI were looking at two "persons of interest."

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Firearms lie on beds beside a broken window of Las Vegas shooter Stephen Paddock's 32nd floor room of the Mandalay Bay hotel in October 2017, in an evidence photo released by the Clark County sheriff.

Las Vegas Metropolitan Police Department via AP

Paddock was dead, but it was possible someone else could be charged with a crime in connection with the deadliest mass shooting in modern U.S. history. Last week, the names emerged – one was Paddock's girlfriend, whom police had cleared, and the other was a man named Douglas Haig of Arizona.

Haig talked to various media, including the AP, and held a news conference characterizing his sale of tracer ammunition to Paddock as a routine transaction allowed by the law.

But Phoenix newsman Jacques Billeaud wasn’t convinced. He called a source he has cultivated in law enforcement. The source was willing to help but didn’t know the answer to Billeaud’s questions.

Then, a few days later, after Haig's news conference, the official called to tell Billeaud that Haig indeed had been charged with a crime. Billeaud quickly checked PACER, the public access to electronic court records system, and found a criminal complaint had just been filed that said armor-piercing ammunition with Haig's fingerprints were found in Paddock's hotel room.

Haig was charged with illegally manufacturing and selling the ammunition. The charge also seemed to contradict Haig's claim that there was nothing suspicious about his dealings with Paddock: The complaint said Haig saw Paddock – the man who would go on to gun down dozens – put on gloves before he touched the ammunition.

Billeaud's source work put the AP ahead on a competitive story.

The lesson: Just because sources can’t deliver the goods on one day doesn't mean they won't have answers in the days to come. Billeaud's relationship with his source put the AP ahead, and local and national customers used the AP as first word on a competitive story. For sticking with the story and using long-term source work to break news, Billeaud will receive this week’s $300 Best of the States prize.